Columbium-hafnium base alloys with yttrium addition



United States Patent Wash. No Drawing. Filed Jan. 4, 1965, Ser. No. 423,375 2 Claims. (Cl. 75-174) This application is a continuation-in-part of our earlier application, Serial No. 330,601, filled December 16, 1963, now;abandoned.

This invention relates to columbium-hafnium base alloys, and more particularly to columbium-hafnium base alloys of the solid solution type which, by the addition of yttrium, exhibit improved characteristics rendering the alloys especially useful for high temperature service.

The present invention constitutes an improvement over the columbium-hafnium base alloys described in the copending application of Edmund F. Baroch et al., Serial No. 269,813, filed April 1, 1963, now abandoned and having common assignees herewith.

The above mentioned copending application discloses columbium-hafnium base alloys of the solid solution type which exhibit many improved characteristics over alloys of the prior art, and it is the principal object of the present invention to improve still further on these characteristics, particularly those characteristics of alloy stability, weld ductility and reproducibility.

The foregoing and other objects and advantages of this invention will appear from the following detailed description.

Although the alloys of the aforementioned copending application have welding characteristics and strength characteristics consistent with the present state of the art, it was desirable to improve the welding characteristics and to increase the high temperature strength by further alloy modifications. It was anticipated that the addition of yttrium would result in a fine dispersion of yttrium compounds which would eflectively dispersion strengthen the alloy and that the addition of yttrium also would serve to scavenge the residual oxygen and nitrogen from the alloy, thereby increasing ductility and Weldability.

Upon the addition of from 0.02 to 0.5 and preferably from 0.05 to 0.3% by weight of yttrium to the alloys of the aforementioned copending application, it was found that the unstable second phase normally found in alloys of the copending application was absent from the microstructure and was replaced by a stable, randomly distributed, agglomerated phase (probably oxides and/ or nitrides of yttrium) which did not respond to'heat treatment.

The dispersion of this stable second phase was such that no strengthening, as was originally anticipated, was observed. Thus, the alloys of the present invention exhibit all the characteristics of solution hardening with consequent improvement in thermal stability, workability, and weld ductility over the alloys of the aforementioned copending application.

Fundamentally, the solid solution type alloys of the present invention include, as essential components, from 3,266,892 Patented August 16, 1966 about 4 to 15% by weight hafnium, at least one of the metals tungsten and molybdenum, or combinations thereof, in an amount equal to from 5 to 15% by weight tungsten, wherein 1% by Weight molybdenum is substantially equal to 2% by weight tungsten, from 0.02 to 0.5% by weight yttrium, and the balance essentially columbium.

For purposes of the present invention, therefore, metals of the highest purity available are employed. Although the metals may be produced and purified by various processes, in the alloy examples set forth hereinafter the metals columbium and molybdenum were purified by electron beam melting of a consumable electrode in .a vacuum furnace; tungsten was provided in a form of powder; and hafnium sponge and yttrium rod were utilized.

It is to be noted here that the hafnium content of the alloys includes a small percentage of zirconium since, as is well known, zirconium occurs naturally with hafnium in the ore and is not completely removed during commercial extraction. In the alloy examples set forth hereinafter, the hafnium content includes about 3% by weight zirconium, characteristic of commercial hafnium currently available. Thus, although the alloy examples contain less than 0.5% zirconium and may contain up to about 2% zirconium without impairing fabricabality to a serious degree, it is to be understood that zirconiumis considered to be an undesirable impurity. Although fabricability of the alloys of the present invention is enhanced by eliminating zirconium, the degree of improvement does not justify the cost of its removal from the commercial hafnium presently available.

It is to be noted further that the elements oxygen, nitrogen and carbon also are considered to be undesirable im purities in the alloys of the present invention, and therefore are to be eliminated or at least minimized to the most practicable degree. In this regard it has been determined that when each of these impurities is held below about 200 parts per million in the ultimate alloy, the latter exhibits characteristics of fabricability and weldability which enable the alloys to be further processed by conventional techniques.

In the metals employed for the alloys of the present invention the basic impurities of oxygen, hydrogen, nitrogen and carbon were determined to total, in parts per million, less than 165 in columbium, 1600 in hafnium, 55 in tungsten, 153 in molybdenum, and 1050 in zirconium. All other metallic impurities were determined to total, in parts per million, less than 2200 in columbium, 450in hafnium, 750 in tungsten, 750 in molybdenum, and 1150 in zirconium.

Production alloys are consolidated by combining electron beam and are melting. A master alloy of columbium and the non-volatile additions, including tungsten and/or molybdenum, is electron beam purified. Volatile additions, such as hafnium and yttrium, are added to the purified master alloy to form a consumable arc melting electrode. Double consumable arc melting insures alloy homogeneity. This technique results in an ingot in which carbon, oxygen and nitrogen contents generally are less than parts per million.

Examples of alloy compositions embodying this invention are set forth in the following Table I, the constituents being indicated in percent by weight:

TABLE I Alloy Hf Y W Mo Ch 4 0. 02-0. 5 Balance. 4 0. 02-0. 5 D0. 4 0. 02-0. 5 10 0. 02-0. D0. 0. 02-0. 5 Do. 10 0. 02-0. 5 Do. 0. 02-0. 5 Do. 15 0. 02-0. 5 Do. 15 0. 02-0. 5 D0. 4 0. 02-0. 5 2. 5 D0. 10 0. 02-0. 5 2. 5 Do. 15 0. 02-0. 5 2. 5 Do. 4 0. 02-0. 5 5 Do. 10 0. 02-0. 5 5 Do. 15 0. 02-0. 5 5 Do. 4 0. 02-0. 5 7. 5 Do. 10 0. 02-0. 5 7. 5 Do. 15 0. 02-0. 5 7. 5 Do. 4 0. 02-0. 5 2. 5 D0. 4 0. 02-0. 5 5 D0. 4 0. 02-0. 5 2. 5 Do. 10 0. 02-0. 5 2. 5 D0. 10 0. 02-0. 5 5 Do. 10 0. 02-0. 5 2. 5 D0. 15 0. 02-0. 5 2. 5 Do. 15 0. 02-0. 5 5 Do. 15 0. 02-0. 5 2. 5 Do.

Test buttons of the alloys listed in Table I and other related alloys were prepared by melting the constituents in a gettered inert atmosphere, using a non-consumable tungsten electrode, and remelting 8 to 12 times to insure maximum homogeneity. Tests conducted on the sample buttons gave indication that the fabricability of alloys having greater than about 15% hafnium is impaired .to an impracticable degree; that greater than about 15% tungsten, or about 7 /2 molybdenum, or proportionate combinations thereof, impairs fabricability; and that greater than about 0.5% yttrium results in undesirable segeration and unstable melting characteristics.

Similarly, alloys containing less than about 4% by weight hafnium exhibit insufficient high temperature strength for purposes of the present invention.

Fabrication tests and tensile tests conducted on the aforementioned samples gave evidence of the superiority of alloy No. 5 for extreme high .temperature utility. The physical characteristics of this alloy are set forth in Table II, wherein the values listed represent averages of several test samples of the alloy. The chemical composition of the test samples includes, in parts by weight, 77.88% columbium, 911% hafnium (the latter including about 3% Zirconium), 9-1l% tungsten and 0.2% yttrium. The basic impurities were determined to total, in parts per million, less than 200 oxygen, less than 150 nitrogen, less than hydrogen, and less than 100 carbon.

It will be noted that this alloy differs from the alloy identified in Table II of the aforementioned copending application only by the addition of 0.2% by weight yttrium.

However, this addition materially improves the Weld ductility of the alloy, as indicated by comparison of the corresponding tabulated data. In this regard welds in the alloy identified in Table II of the aforementioned copending application are characterized by a minimum room temperature bend radius of ST (90 and a ductile-brittle transition temperature (4T 90 bend) of F. The improvements in these respects in the present alloy are self evident. On the other hand, the compared alloys are quite similar in the characteristics of density, recrystallization, modulus of elasticity and coatability.

The microstructure of alloy No. 5 is characterized by a randomly distributed dispersed phase in both cast and Wrought conditions. This phase is not affected by normal heat treatment at least as high as 3000 F. and is observed in both the heat affected and fusion zones of welds. In contrast, the microstructure of alloy No. 2 of the aforementioned copending application exhibits an unstable second phase which precipitates in grain boundaries and subgrain boundaries in weld metal and cast ingots and in bands in the wrought product. The stability of the dispersed phase of alloy No. 5 results in a finer grain size in the weld fusion zone and less grain growth in the heat affected zone.

Although the effects of the addition of yttrium are not completely understood, it is believed that these alloys possess good strength at high temperatures, better weld ductility and finer microstructure, because of the extreme activity of these additives as an oxygen and nitrogen getter, to form complex oxides and nitrides which occur randomly dispersed in the cast structure. With the oxygen and nitrogen impurity content tied up in this manner, there is prevented the formation of oxides of tungsten, columbium or hafnium which normally concentrate in grain boundaries and thus materially reduce the ductility of the alloy. In this respect the dispersed complex oxides and nitrides of yttrium do not lower the ductility of the alloy. Still further, these oxides and nitrides of yttrium have high melting points and are very stable, and accordingly do not break down and go back into solution when exposed to lu'gh temperatures during service. However, this agglomerated dispersed phase apparently is too coarse to function as a dispersion hardner, as evidenced by the slight lowering of the strength of the alloy.

It will be apparent to those skilled in the art that various modifications of the details described hereinbefore may be made without departing from the spirit of this invention and the scope of the appended claims.

Having now described our invention and the manner in which it may be used, what we claim as new and desire to secure by Letters Patent is:

1. A columbium-hafnium base alloy of the sol-id solution type, consisting essentially of 9-11% by weight hafnium, 9-11% by weight tungsten, 0.050.3% by weight yttrium, and the balance essentially columbium.

2. A columbium-hafnium base alloy of the solid solution type, consisting essentially of 10% by weight hafnium, 10% by weight tungsten, 0.2% by weight yttrium, and the balance essentially columbium.

References Cited by the Examiner UNITED STATES PATENTS 2,973,261 2/1961 Frank 75-174 3,056,672 10/1962 Clark 75 174 3,156,560 11/1964 Semmel 75 174 OTHER REFERENCES Semmel, J. W., Jr., The Effect of Rare-Earth Metal Additions on the Ductility of Arc-Melted Group Va Metals, Report No. 57-RL-1736, April 1957, published by General Electric Research Laboratory, The Knolls, Schenectady, New York (12 pages).

DAVID L. RECK, Primary Examiner.

W. C. TOWNSEND, Assistant Examiner. 

1. A COLUMNIUM-HAFNIUM BASE ALLOY OF THE SOLID SOLUTION TYPE, CONSISTING ESSENTIALLY OF 9-11% BY WEIGHT HAFMIUM, 9-11% BY WEIGHT TUNGSTEN, 0.05-0.3% BY WEIGHT YTTRIUM, AND THE BALANCE ESSENTIALLY COLUMBIUM. 